Processors for signal conditioning and arithmetic operation are the core types of LSI for modern signal processing technology. In recent years, these processors were required to achieve both lower power consumption and higher operation speed. Phase locked loop circuits included in such processors, therefore, have to generate clock signals ranging from extremely high speed to extremely low speed during, for example, an intermittent operation. Since such processors are applied to a wide range of applications, input signals of an extremely wide range of frequency are given to a phase locked loop circuit. A phase locked loop circuit included in such processor, thus, desirably has a function to automatically modify a response constant depending on the frequencies of input and output signals. A phase locked loop circuit having such function is particularly called an autonomously controlled phase locked loop circuit.
Natural frequency ωon (also known as a loop bandwidth or a response frequency) and damping factor ζ of a phase locked loop circuit are generally expressed by Formulas (1) and (2) below, respectively.
                    Mathematical        ⁢                                  ⁢        Expression        ⁢                                  ⁢        1                                                                      ω          ⁢                                          ⁢          n                =                              KoIp                          2              ⁢              π              ⁢                                                          ⁢              C                                                          (        1        )                                ζ        =                              CR            2                    ⁢          ω          ⁢                                          ⁢          n                                    (        2        )            
wherein Ko denotes the gain of a voltage controlled oscillator, Ip denotes the magnitude of a charge current outputted from a charge pump circuit, C denotes the capacitance value of a loop filter, and R denotes the resistance value of the loop filter. Formula (3) below holds for the relationship between input frequency ωin and natural frequency ωn.
Mathematical Expression 2ωin=αωn  (3)
wherein α is the proportionality constant.
In a phase locked loop circuit, proportionality constant α, which is a relationship between input frequency ωin and natural frequency ωn, and damping factor ζ are preferably constant regardless of input frequency ωin. A conventional autonomously controlled phase locked loop circuit controls the magnitude of a charge current and the time constant of a loop filter corresponding to the control voltage of a voltage controlled oscillator to maintain the proportionality constant and the damping factor at predetermined values with respect to a frequency variation of an input signal (for example, refer to Patent Document 1).
Patent Document 1: U.S. Pat. No. 5,727,037